1. Field of the Invention
The present invention relates to a paper feeding apparatus such as an automatic document feeder used for an image forming apparatus such as a copying machine.
2. Description of the Related Art
As a prior art for detecting the thickness of a paper sheet being conveyed, a technique is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-127327, in which the displacement of a feeding roller is detected. An embodiment in this official gazette uses a non-contact type minute displacement sensor (e.g., a laser displacement sensor available from Keyence Corporation), which is very expensive and hence is not practical.
The object in detecting the thickness of a paper sheet is to reliably convey a paper sheet by adjusting the pressing force of a pressing roller at a paper feeding section for reversing an original.
An embodiment is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-52780, in which double or multiple paper-conveying of sheets is detected, and different paper discharge portions are respectively arranged for sheets which have undergone multiple paper-conveying, and for the remaining normally conveyed sheets, thereby separately processing only sheets which have undergone multiple paper-conveying.
Jpn. Pat. Appln. KOKAI Publication No. 3-107174 discloses an embodiment associated with an automatic original conveying apparatus for conveying an original to a copying machine, in which originals which have undergone a copying operation are counted and displayed on the basis of detection signals from a paper discharge sensor of the automatic original conveying apparatus.
In the first prior art, pressure adjustment is performed only to improve the reliability in conveying a paper sheet. In addition, the means for measuring/detecting the displacement of the feeding roller is expensive and hence is not practical.
In the second prior art, the following problems are posed in the method of detecting the displacement amounts of the convey rollers by using the displacement sensor.
In this embodiment, it is described that the resolution of the displacement sensor is about 1 to 10 .mu.m. In addition, since the precision of the outer diameter of each roller for conveying a sheet is 5 to 10 .mu.m, fluctuations in displacement data are large before any displacement of a sheet is detected. Therefore, it is difficult to detect a multiple paper-conveying error in practice.
In the use of the means for detecting the thickness of an original by using the capacitance sensor, when the capacitance of a conveyed sheet is low, or the humidity is high, the thickness detection result obtained with respect to the same original differs from that obtained in a normal condition.
When a document is linearly set, the thickness of the document can be accurately expressed by a change in capacitance. If, however, an original is curled, or the degree of curling of an original changes between the electrodes of the detector, it is difficult to accurately detect the thickness of the original.
Assume that trial reading of originals is performed once, and multiple paper-conveying errors are detected on the basis of the resultant thickness and length data. In this case, if the frequency of multiple paper-conveying errors is different from that in the trial read operation, errors may occur in determination of multiple paper-conveying.
In the third prior art, the number of originals which have undergone a copying operation is counted by the paper discharge sensor of the automatic original conveying apparatus to be displayed. In this case, a user must count the number of originals set on the automatic original conveying apparatus in advance, and compare the count value with the displayed count value, thereby detecting a multiple paper-conveying error by himself or herself. It is apparent that when a large quantity of documents are to be copied, communicated, or stored in a medium such as an optical disk, this method imposes a large load on the user.
For an apparatus for feeding a paper sheet, especially an original, it is very important to detect a multiple paper-conveying error. An automatic paper feeding apparatus is used for a facsimile apparatus, an optical disk file apparatus, and the like as well as a copying machine.
In a facsimile apparatus or an optical disk file apparatus designed to convert original image information into an electrical signal, a multiple paper-conveying error poses a serious problem. In a facsimile apparatus, a multiple paper-conveying error cannot be determined from copied paper sheets, and omission of information due to a multiple paper-conveying error is often informed upon an inquiry from the user at the destination. This situation is inconvenient for the user at the destination, and facsimile communication must be repeated.
In an optical disk file apparatus, when image information is converted into an electrical signal and stored in a recording medium such as an optical disk, in order to check a multiple paper-conveying error, the stored information must be read from the optical disk to be checked on a display means such as a CRT. This greatly reduces the effect of office automation (OA) based on an optical disk file apparatus capable of storing a large amount of information.
As described above, the importance of detection of a multiple paper-conveying error has been recognized. However, no attempts have been made to devise a means for realizing such detection.